How do Ottawa painters deal with mould on walls before applying new paint?

Mould on interior walls is a serious issue that Ottawa painters encounter regularly — and the absolute worst thing you can do is paint over it. Painting over mould doesn't kill it, doesn't seal it in, and creates a worse problem that will cost significantly more to fix later. Here's how professionals handle it properly.

Why Mould Is So Common in Ottawa Homes

Ottawa's climate creates a perfect storm for interior mould growth:

• Condensation on cold walls — When outdoor temperatures drop to -20C or -30C, exterior walls and especially window frames become cold enough to cause indoor moisture to condense. This persistent dampness feeds mould growth from November through March
• Ice damming — Ottawa's heavy snowfall (200+ cm annually) combined with heat loss from roofs causes ice dams that force water into wall cavities and ceilings
• Basement moisture — High water tables in parts of Orleans, Barrhaven, Kanata, and riverside neighbourhoods push moisture through foundation walls
• Summer humidity — July and August humidity regularly exceeds 70%, promoting mould growth on surfaces that got damp during winter
• Poor ventilation — Older Ottawa homes often lack adequate bathroom exhaust fans, and many homeowners seal up their homes tightly for winter, trapping moisture inside

The Professional Mould Remediation Process

Step 1: Identify and Fix the Moisture Source

This is non-negotiable. No amount of cleaning or mould-resistant paint will matter if the moisture source isn't addressed. Common culprits in Ottawa homes:

• Bathroom without proper exhaust fan (or fan venting into attic instead of outside) — very common in pre-1990 Ottawa homes
• Poor exterior caulking/flashing allowing water infiltration
• Foundation wall seepage — especially during spring snowmelt
• Condensation from insufficient insulation on exterior walls
• Leaking plumbing hidden in wall cavities

A painter who jumps straight to cleaning without asking about the moisture source is a red flag. The Ontario Building Code requires that moisture problems be corrected before refinishing interior surfaces.

Step 2: Assess the Extent of Mould

Small areas (less than 10 square feet) can generally be handled by a painter or homeowner with proper precautions. Larger areas, or mould inside wall cavities, should involve a certified mould remediation specialist. In Ottawa, professional mould assessment costs $300 to $600, and remediation ranges from $500 to $5,000+ depending on extent.

The Ontario Ministry of Labour considers mould a workplace hazard. Professional painters with WSIB coverage are trained to protect themselves and your family during mould-related work.

Step 3: Containment and Protection

For any visible mould treatment:

• N95 respirator mask — standard surgical masks are NOT adequate for mould spores
• Plastic sheeting over doorways to contain the work area and prevent spores from spreading to other rooms
• Gloves and eye protection
• HEPA-filtered air scrubber for larger areas

Professional Ottawa painters who deal with mould regularly keep this equipment on their trucks. Budget an extra $100-$200 for containment setup on a typical mould remediation paint job.

Step 4: Mould Removal

The mould itself must be physically removed, not just killed. Dead mould spores are still allergenic and can cause health reactions. The process:

Spray affected area with a mould-killing solution. Professional options include concrobium mould control or a diluted bleach solution (1 part bleach to 10 parts water). Concrobium is preferred by most Ottawa painters because it doesn't damage paint, won't bleach surfaces, and is less toxic. It costs $12-$18 per ready-to-use bottle

Scrub the surface with a stiff brush to physically remove mould growth

Wipe clean and allow to dry completely — 24-48 hours minimum. In Ottawa's winter, when indoor air is dry from forced-air heating, this can go faster. In summer humidity, use a dehumidifier to speed drying

Repeat treatment if any mould remains visible

If mould has penetrated drywall (common when moisture exposure has been prolonged), the affected drywall must be cut out and replaced. You cannot clean mould out of drywall paper facing once it's infiltrated — the paper is literally mould food. Drywall replacement adds $3 to $6 per square foot including taping and mudding.

Step 5: Priming With Mould-Blocking Primer

After the surface is clean and completely dry:

• Zinsser Mold Killing Primer is the industry standard — it's EPA-registered as a mould-killing agent and costs $35-$50 per gallon in Ottawa. It bonds to both previously mould-affected surfaces and new drywall patches
• Kilz Mould & Mildew Primer is another solid option at $30-$45 per gallon
• Apply a full coat over the entire affected wall, not just the mould spot — spores spread beyond the visible growth area

Step 6: Finish Paint With Mould-Resistant Properties

For rooms prone to moisture (bathrooms, kitchens, basements), use a paint specifically formulated with mould and mildew inhibitors:

• Benjamin Moore Aura Bath & Spa — excellent mould resistance, designed for high-humidity rooms. $75-$85 per gallon in Ottawa
• Sherwin-Williams Emerald — contains antimicrobial agents that prevent mould growth on the paint film. $70-$80 per gallon
• Any premium paint with a mould-resistant additive mixed in (available at most Ottawa paint stores for $5-$10 per gallon of additive)

Prevention Going Forward

After dealing with mould, Ottawa homeowners should:

• Maintain indoor humidity between 30-50% year-round (use a hygrometer — $15-$25 at any hardware store)
• Run bathroom exhaust fans for 20 minutes after every shower
• Ensure dryer vents exhaust outside the home
• Check for condensation on windows during cold snaps — persistent condensation means excess indoor humidity
• Address any new water stains immediately — they don't fix themselves

Cost Summary for Mould-Related Paint Prep

| Service | Typical Ottawa Cost |
|---------|--------------------|
| Mould assessment | $300-$600 |
| Small area treatment + paint prep | $200-$500 |
| Drywall replacement (if needed) | $3-$6/sq ft |
| Mould-killing primer | $35-$50/gallon |
| Mould-resistant finish paint | $70-$85/gallon |
| Professional remediation (large area) | $500-$5,000+ |

Find experienced Ottawa painters through the Ottawa Construction Network directory — look for those who specifically mention mould remediation experience. Ottawa Paint Contractors' Paint IQ has more guidance on moisture-related painting challenges specific to our climate.